Q What is Dual Tone Multi Frequency?
Dual Tone Multi Frequency (DTMF) was first introduced in 1963 with 10 buttons in Western Electric 1500 -type telephones. DTMF was originally known as Touchtone. DTMF is a more efficient means than dial pulsing for transferring telephone numbers from a subscriber's location to central office switching machine.
DTMF is a simple two-of -eight encoding scheme where every digit is represented by linear addition of two frequencies. DTMF is strictly for signalling between a subscriber's location and the nearest telephone office or message switching centre. DTMF is many times confused with another two-tone signalling system known as multi frequency signalling (Mf) that is a two-of-six code designed to be used only to convey information between two electronic switching machines. Figuredemonstrates four-row-by-four column keypad matrix used with a DTMF keypad. As the figure demonstrates, keypad is comprised of 16 keys and eight frequencies. Most household telephones, though, aren't equipped with special-purpose keys located in fourth column (which implies A, B, C, and D keys). Thus most household telephones actually use two-of-seven tone encoding scheme. Four vertical frequencies (known as low group frequencies) are 697 Hz, 852 Hz, and 941 Hz and four horizontal frequencies (called high group frequencies) are 1209 Hz, 1336 Hz,1477 Hz, and 1633 Hz. Frequency tolerance of the oscillators is + .5%. As demonstrated in Figure, digits 2 through 9 canalso be used to represent 24 of the 26 letters (Q and Z are omitted). Letters were originally used to identify one local telephone exchange from another.